The present invention relates to a process for converting secondary products formed during the thermal cracking of 1,2-dichloroethane in the preparation of vinyl chloride.
The current industrial processes for preparing vinyl chloride are based on the thermal cracking of 1,2-dichloroethane. Generally, 1,2-dichloroethane is obtained, either by liquid-phase chlorination of ethylene or by gas-phase oxychlorination of ethylene using a gas containing oxygen and derived from the thermal cracking of 1,2-dichloroethane and/or from any other source. The thermal cracking of 1,2-dichloroethane (referred to hereinbelow as DCE) is generally carried out at temperatures ranging from 300.degree. C. to 650.degree. C. and at pressures of between 8 bar and 40 bar.
A certain number of by-products liable to be found in DCE from cracking originate either from the products of the thermal dissociation, which are recycled, or from products resulting from the manufacture of the DCE.
The by-products are of a nature to give rise in particular to coking of the cracking oven faster than in the case of pure DCE. This coking results in an increase in the loss of charge in the reaction zone. Consequently, the production must be stopped frequently and this zone cleaned. Moreover, the presence of coke on the walls of the reactor results in an increase in temperature which may be detrimental to the strength of the equipment.
These by-products are classified herein as light byproducts and heavy by-products.
The term light by-products, referred to hereinbelow as the lights, is understood in the present text to denote products having a boiling point below 83.7.degree. C., and the term heavy by-products, referred to hereinbelow as the heavies, is understood in the present text to denote products having a boiling point above 83.7.degree. C.
Among the lights, mention will be made of saturated and unsaturated, optionally chlorinated aliphatic hydrocarbons such as 1,3-butadiene, acetylene, 2-chloro-1,3-butadiene, chloroprene, 1,1-dichloroethylene (trans), 1,1-dichloroethane, vinylidene chloride, chloroform, carbon tetrachloride and 1,1,2-trichloroethylene. Aromatic hydrocarbons such as benzene may also be detected.
Certain lights are difficult to separate from the unconverted DCE since their boiling points are very close to the boiling point of DCE (b.p..sub.760 =83.7.degree. C.) or alternatively because thay form azeotropes with the DCE. This is the case in particular for 1,1,2-trichloroethylene, which is virtually inseparable from DCE by standard distillation means.
In general, the products obtained from the thermal cracking are separated by successive distillations.
Thus, hydrogen chloride which may be used for the oxychlorination of ethylene, is recovered at the head of a first column. Vinyl chloride is recovered at the head of a second column.
Lights are recovered at the head of a third column and heavies and a large amount of DCE are recovered at the foot of the third column.
In general, the heavies undergo a further distillation which makes it possible to recover the unconverted DCE which is of sufficient purity to introduce it into the thermal cracking zone.
As regards the lights, many processes are known for removing them from the unconverted DCE.
Thus, according to British patent GB 938,824, the dissociation products, consisting of vinyl chloride hydrogen chloride, unconverted DCE and by-products, are quenched, HCl and the vinyl chloride are separated out by distillation as mentioned above and the lights, which are necessarily mixed with some of the unconverted DCE, are then subjected to elimination by distillation at the head of a column referred to as the column for fractionating the low-boiling products, and the distillation product is discarded. The main amount of the unconverted DCE is eliminated by distillation in a column for fractionating high-boiling products in order to separate out the high-boiling product. The disadvantage of this process is that the low-boiling products can only be eliminated from the cycle at the expense of large losses of DCE.
According to a process described in patent FR 2,038,347, the lights are converted into high-boiling products, that is to say above 83.7.degree. C., and the pure DCE is extracted by distillation from the high-boiling products. This process is characterized in that the impure DCE is charged with chlorine gas, in the presence of catalysts which are common for the chlorination of ethylene to DCE, at temperatures between 30.degree. C. and 85.degree. C. Preferably, the process is performed in the presence of 50 to 500 ppm of FeCl.sub.3, relative to the DCE, as catalyst. It is possible to proceed such that the impure DCE is conveyed to a plant used for the reaction of chlorine and ethylene with formation of DCE. The ethylene chlorine and the impure DCE are charged therein together in the presence of the catalyst. The catalyst is separated out by washing with water and the pure dichloroethane is eliminated by distillation from the high-boiling impurities.
According to a second variant, the process may be carried out so that the impure DCE is liberated, by distillation, from the low-boiling products. The total or partial amount of the concentrate of low-boiling products accumulating at the head of the column for fractionating the low-boiling products in question is removed continuously. The concentrate is charged with chlorine gas in the presence of the catalyst and the mixture containing the catalyst is conveyed to the column for fractionating the low-boiling products. The product at the bottom of the column for fractionating the low-boiling products is transferred continuously into a column for fractionating the high-boiling products and the pure DCE is removed by distillation from the high-boiling products containing the catalyst.
In a third variant, the process may be performed so that the impure DCE is liberated, by distillation, from the low-boiling products, the total amount or some of the concentrate of the low-boiling product accumulated at the head of the distillation column is removed continuously, and the concentrate is charged with chlorine gas in the presence of the catalyst. In the third variant, the mixture containing the catalyst is conveyed to the lower part of the column, the pure DCE below the head of the column is removed continuously and the high-boiling products are discharged from the bottom of the column.
Although effective for lowering the contents of certain by-products, these various techniques do, however, have certain drawbacks.
The fact that the lights are introduced into the zone for the preparation of the DCE by direct chlorination of ethylene makes it necessary to work with a large excess of chlorine, thereby entailing expensive subsequent treatments in order to destroy the unconsumed chlorine.
Moreover, the chlorination of the lights in the reactor for the chlorination of ethylene results in a considerable loss of selectivity for the reaction EQU Cl.sub.2 +C.sub.2 H.sub.4 .fwdarw.C.sub.2 H.sub.4 Cl.sub.2
which is characterized by a very substantial increase in the formation of 1,1,2-trichloroethane.
The chlorination of the lights as proposed in the abovementioned variants 2 and 3 requires additional apparatus, intricate implementation for effectively controlling the chlorination reactions, and the introduction of catalyst which will need to be purged thereafter.